1,2,4-triazine derived binuclear lead(ii) complexes: synthesis, spectroscopic and structural investigations and application in organic light-emitting diodes (OLEDs)

Two novel binuclear complexes of Pb(ii) were synthesized by reacting a 3-(2-pyridyl)-5-(4-methoxyphenyl)-1,2,4-triazine (PMPT) ligand with different anionic co-ligands (1: bromide, 2: acetate and isothiocyanate) in a 1 : 1 molar ratio of PMPT ligands to lead(ii) salts. The complexes, [Pb2(μ-PMPT)2Br4] (1) and [Pb2(μ-PMPT)2((μ-CH3COO)2(NCS)2] (2), were characterized using various physicochemical techniques such as CHN analysis, FT-IR spectroscopy, and 1H NMR spectroscopy. Additionally, their structures were determined using single-crystal X-ray diffraction. Based on the obtained structural parameters, complex 1 exhibited a PbN3Br2 environment, while complex 2 displayed a PbN4O3 environment, with holodirected and hemidirected coordination spheres, respectively. Within the crystal network of the complexes, there were interactions involving C–H⋯X (X: O, S, N) as well as π–π stacking. The Pb(ii) complexes were further investigated for their potential use as the emitting layer in organic light-emitting devices (OLEDs). The current–voltage and luminescence-voltage characteristics, as well as the electroluminescence (EL) properties of the complexes, were studied.


Materials and measurements
All starting materials and solvents were purchased from commercial sources (Merck and Sigma-Aldrich) and used directly without any purication.The PXRD patterns were measured using a X-ray diffractometer (D500 S) utilizing Cu Ka (l = 0.15418 nm) radiation source (30-40 kV and 40-50 mA) in the range of 2q = 4-50°.The infrared spectra in the range 4000-400 cm −1 were recorded as KBr pellets with a FT-IR 8400-Shimadzu spectrometer. 1 The tube was then sealed and the ligand-containing arm was immersed in a bath at 65 °C while the other arm was maintained at ambient temperature. 24Aer a week, yellow crystals that were deposited in the cooler arm were ltered and dried in air.Yield: 0.

Crystal structure determination
Crystallographic data are given in Table 1.Single-crystal X-ray diffraction measurements were performed on a STOE IPDS 2T image plate diffractometer system equipped with a sealed Mo Xray tube and a graphite monochromator crystal (l(Mo-K a = 0.71073 Å).Data reduction and numerical absorption correction were done with STOE X-AREA soware. 25All structures were solved by direct methods using SHELXS-2018 and rened with SHELXL-2018 (ref.26) using WinGX 27 as a graphical frontend.All non-hydrogen atoms were rened with anisotropic thermal.Hydrogen atoms were included on idealized positions applying the riding model.Olex2 soware 28 and Diamond 3.2k 29 were used for structural analyses and visualization.CCDC 2350383 and 2350384 contains the supplementary crystallographic data for this paper.These data can be obtained free of charge via https://www.ccdc.cam.ac.uk/structures/.

Fabrication of OLEDs
ITO, were used as the conducting anodes.PEDOT: PSS as a hole injection layer was spin-coated on clean ITO substrate of 55 nm thickness and baked in an oven for 1 hour at 1200 °C.Aerward, PVK as a hole-transporting material and PBD as an electrontransporting material were doped with compound 1 and 2. PVK, PBD, and compound 1, 2 at ratio of 100 : 40 : 8 were blended in dimethylformamide (DMF) and then spin-coated and baked at 80 °C for 1 h.Finally, Al was evaporated by thermal evaporation method, respectively.Fig. 1 shows a schematic structure of the devices.

Infrared spectra of complexes
The infrared (IR) spectra of compounds 1 and 2 were analyzed, and the selected frequencies are provided in Section 2. In the examined complexes, the stretching vibrations of the C]C, C] N, and N]N groups were observed at signicantly lower values compared to the vibrations of n(C]N) and n(N]N) in the 3-(2-  pyridyl)-5-(4-methoxyphenyl)-1,2,4-triazine (PMPT) ligand.This observation supports the coordination of the triazine and pyridyl rings to the metal ion. 30The weak band observed at 2950-3000 cm −1 is attributed to the methoxy group of the PMPT ligand and the acetate n(CH) mode in compound 2. In 2, the bands around 1576 and 1412 cm −1 correspond to the modes n asym (COO) and n sym (COO), respectively, indicating the presence of the acetate ligand in the molecule.Previous attempts have been made to correlate the positions of these modes or the frequency difference Dn (n asym − n sym ) with the bonding type. 31he Dn value of acetate in the lead(II) complex is 164 cm −1 , which aligns with the expected bidentate and bridging coordination of acetate.Additionally, the bands at a frequency of 2049 cm −1 in the IR spectrum of 2 provide evidence of Ncoordination between the terminal isothiocyanate anions and the lead center, as supported by the crystal structure 32 (Fig. S1 and S2 †).
3.1.1. 1H NMR spectroscopy of complexes.The 1 H NMR spectra of all compounds were obtained in DMSO-d 6 and clearly show the presence of the PMPT ligand.In the spectrums, peaks ranging from 7.65 to 10.13 ppm conrm the aromatic nature of the ligand.A single peak at the lowest magnetic eld (10 ppm) corresponds to the hydrogen atom in the triazine ring (H a ).The signal at 8.8 ppm can be attributed to the hydrogen near the pyridine nitrogen atom (H b ).The peaks representing the other hydrogen atoms in the pyridine ring appear between 7.47 and 8.90 ppm, manifesting as two doublets of doublets (H d and H e ) and one doublet (H c ).The hydrogen atoms in the phenyl ring are observed as two doublets at 7.15 and 8.4 ppm (H f and H g ).A singlet peak at 3.8 ppm corresponds to the three hydrogen atoms in the methoxy group (H h ).In the spectrum of compound 2, in addition to the mentioned peaks, a singlet peak at the highest magnetic eld (1.69 ppm) is observed, which represents the hydrogen atoms of the acetate anion (Fig. S3 and S4 †).

Crystal structure description
The crystalline phase purities for 1 and 2 were conrmed by the PXRD patterns.Fig. S5 † shows that the powder patterns of the two complexes match quite well with those simulated from single-crystal X-ray data, indicating the bulk purities of the complexes.
Scheme 2 illustrates the various possible coordination modes of derivatives of 3-(2-pyridyl)-1,2,4-triazine (PTZ) ligands.The PTZ ligands can form mononuclear lead complexes through a bidentate coordination site similar to 2,2 0 -bipyridine (referred as form A 1 ).9][20][21] By combining bidentate and monodentate coordination sites provided by the PTZ ligands, it is possible to obtain dinuclear complexes (referred to as forms C 2 and D 2 ).Only one binuclear complex of silver(I) in coordination form D 2 has been reported. 33However, there are no examples in the (1) crystallizes in the triclinic space group P 1 (Table 1), with one formula unit per unit cell.A molecular view and selected bond parameters of 1 are presented in Fig. 2 and Table 2. Considering the PMPT ligand as a bidentate and monodentate N-donor ligand, along with two bromide anions, the compound [Pb 2 (m-PMPT) 2 Br 4 ] suggests the presence of a ve-coordinate Pb atom.This coordination number is relatively low for Pb(II) in an N and Br-donor environment.The structure solution reveals the Pb atoms occurring in pairs that are approximately 3.900(2) Å apart.These pairs are formed through N-bridging in a centrosymmetric dimer unit consisting of PbN 3 Br 2 entities.The structure of this "dimer" unit is quite remarkable, as depicted in Fig. 3, displaying a highly "hemidirected" coordination sphere.This observation suggests that this system might serve as an example of a stereochemically active lone pair.Within a "hemidirected" coordination sphere, the length of the Pb-X bonds varies and is generally greater than 0.4 Å.This variation is inuenced by their proximity to a stereochemically active lone pair or their distance from it. 34However, in the case of compound 1, there is an approximate difference of 0.2 Å in the lengths of the ve Pb-N and Pb-Br bonds.Notably, these longer bond lengths are indicative of a "holoidirected" coordination sphere.
Upon further examination, it becomes clear that the interpretation of the crystal structure mentioned above is an over-simplication.The chains of Pb atom pairs, aligned parallel to the crystallographic c-axis, arise from close "intermolecular" contacts between such dimeric units.This arrangement is illustrated for a single adjacent pair in Fig. 3.While the parallel alignment of certain phenyl groups may initially suggest the presence of p-stacking interactions throughout the lattice, it is actually a consequence of two-hapto interactions occurring between the Pb atoms and phenyl groups originating from separate dimers.
A search was made generally for Pb/C approaches and it appears that Pb atoms in compound 1 may also be involved in h 2 interaction with the phenyl groups of another dimer.Thus, the Pb atoms are linked to two carbon atoms of phenyl groups, with distances Pb1/C14 ii and Pb1/C13 ii of 3.465(6) and 3.538(6) Å, respectively.Hence, the Pb II coordination sphere is completed and rather than a PbN 3 Br 2 coordination sphere, the complex can be considered to contain a dihapto interactions (PbC 2 N 3 Br 2 ) center with an irregular seven coordination number but ''holodirected" coordination sphere (Fig. 3a and b).The reported Pb/C separations range is  37 Thus, Pb/ C interactions in compound 1 appear to be yet another factor which can make varying contributions to the stability of complexes of this metal ion.
Within the crystal structure of complex 1, adjacent complex molecules are connected through robust intermolecular hydrogen bond interactions.The primary intermolecular interactions observed in compound 1 include C-H/O and C-H/Br hydrogen bonds, as well as p/p stacking interactions between the molecules.These interactions play a crucial role in determining the overall arrangement of the crystal packing and contribute to the stabilization of the crystal structure in a twodimensional supramolecular manner.For further details, refer to Fig. S6 † and Table 3.
In compound 2, the structure may be considered as a coordination polymer of lead(II) consisting of dimeric units with a building block of [Pb 2 (m-PMPT) 2 (m-CH 3 COO) 2 (NCS) 2 ] (2).Similarly to compound 1, Two PMPT ligand doubly bridge two lead(II) ions via the N atoms (nitrogen of pyridine and triazine as chelating and another nitrogen of triazine as monodentate to another lead atom).The dimeric units are further linked across a center of inversion by two acetate anions, resulting are shown in Fig. 4. The Pb/Pb distances within the 2 moieties, those bridged by the acetate anions are 4.027(3) Å.Within the dimer unit, the carboxylate moiety of each acetate ligand acts as both bidentate, and bridging group (totally tridentate) in a m-1,3 mode: both oxygen atoms of the carboxylate group coordinate to a lead(II) ions yielding the Pb 2 O 2 core.Isothiocyanate anions as acts monodentate and as terminal N donor.Thus per lead atoms in 2 is seven-coordinated (PbN 4 O 3 ) by two nitrogen of two PMPT ligands, nitrogen of isothiocyanate anion and three  2).This arrangement created a gap or hole in coordination geometry around the metal ions (presence of gap is clear), occupied possibly by a "stereoactive" lone pair of electrons on lead(II), and the coordination sphere is hemidirected.The bond length difference more than 0.4 Å in the coordination sphere and the observed shorting of Pb-O bonds on the side of Pb 2+ ion opposite to the putative lone pair supports the presence of the lone pair electrons. 34his particular environment provides a suitable space for forming bonds with other atoms.To explore potential donor centers, it is necessary to extend the bonding range.Within a limit of 3.5 Å (which is smaller than the van der Waals radius), there are Pb/S(thiocyanate) tetrel bonds observed in the crystallographic [010] direction.These bonds have distances of 3.441(2) Å, connecting the dimers and forming a polymeric chain.These distances fall within the sum of the van der Waals radii 38 of the corresponding atoms.The Pb/S tetral distance in compound 2 is similar to that reported for lead(II) complexes with thiocyanate (Fig. 5). 39Additionally, the Pb/Pb distances within the [Pb 2 (m-SCN) 2 ]n moieties measure 6.716 Å.
In compound 2, various types of interactions, including intermolecular, intramolecular, and p-p stacking interactions, contribute to the arrangement of the complex in the crystal lattice (refer to Table 3).To investigate the presence of weak directional intermolecular interactions in 2, Mercury programs

Optical characteristics
The UV-Vis absorption spectra of complexes in ethanol solution are shown in Fig. 6.Compound 1 exhibit two absorption peaks at 380 nm and 360 nm.The UV-Vis spectrum of compound 2 is red shied with respected to compound 1.All peaks can be attributed to the p-p* transitions of the aromatic ligand.Fig. 7 and 8 shows the characteristics of the PL solution and solid state of compounds in the solution state by exciting with a wavelength of 405 nm, the PL spectra of compounds shows peak emission spectrum centered at 596 nm, but in the solid state, the PL spectra of compounds blue shied about 65 nm with respect to solution.
The PL quantum yields of the compounds were achievement using the equation established by Parker and Rees. 42,43The highest PL quantum yield for compounds 1 and 2 are measured as 0.67, and 0.38, respectively.The Förster radius energy transfer rate is commonly employed to showcase the power efficiency in OLED technology.The Förster radius is dened as: Here, k 2 , N A , Q D , n, F D (l) and 3 A (l) represent the orientation factor, Avogadro's number, donor quantum yield, refractive index, uorescence intensity, and extinction coefficient.The Förster radius values for compounds 1 and 2 are measured as 3.34 nm and 4.67 nm, respectively.Table 4 shows the optical property of compounds.With applying of voltage, electrons (e) and holes (h) injected in the PEDOT:PSS and PBD layer, Finally e/h recombines at the compound molecules.Also, the EL intensity at 560 nm (Fig. 11) dependent on the applying voltage and with increasing voltage the EL intensity increases.
Fig. 12 shows the current density of the devices.Turn on voltage of the OLEDs is lower than 4.5 V.With increasing of voltage the current density increases.The J-V characteristics change from ohmic region to the space charge limited current (SCLC) region, respectively.[46][47] Fig. 13 shows luminescence efficiencycurrent density of devices.The differences between the luminescence are ascribed to the role of the PbN 3 Br 2 to PbN 4 O 3 in compounds.Also, with Fig. 7 The PL of solution of compounds.
Fig. 8 The PL of solid state of compounds.Fig. 11 The EL spectrum at various voltages.
the increase in applied current density the luminescence remains reliably stable.

Conclusion
In conclusion, we report the synthesis and physicochemical structural characterization of two new binuclear Pb(II) complexes with PMPT ligand and auxiliary anionic co-ligands: bromide in 1, acetate and isothiocyanate in 2. In all complexes, per PMPT ligand coordinate two lead atoms by three nitrogen atoms (new coordination mode in lead complexes).
Based on the obtained structural parameters, complex 1 exhibited a PbC 2 N 3 Br 2 environment, while complex 2 displayed a PbN 4 O 3 S environment, with holodirected and hemidirected coordination spheres, respectively.It seems that anions play an important role in the activity of lead(II) coordination sphere by creating various intermolecular interactions.Upon closer examination, it becomes evident that tetrel bonding, although not adequately recognized until now, plays a signicant role in the study of lead's solid-state chemistry.This newfound understanding has the potential to greatly aid in the manipulation and design of supramolecular architectures and organometallic frameworks that rely on lead coordination complexes.The prepared complexes show high luminescence efficiency at room temperature and have good stability, which make it suitable for the fabrication of OLEDs.It should be noted that auxiliary ligands are a factor for changing the optical properties of compounds.The results of this work show that these compounds can be used as a precursor in the manufacture of optical devices.
Fig. 12 The current density-voltage characteristic of device.

Fig. 1
Fig. 1 Schematic structures of the device.

Fig. 2
Fig. 2 Molecular structure of compound 1 with atom numbering scheme.

Fig. 9 shows
The EL spectra of OLED devices.The EL of the compound 1 and 2-based devices showed a band in the green and yellow regions, respectively.The EL spectra of the OLEDs depended on the PbN 3 Br 2 , 1 and PbN 4 O 3 , 2 of the environment of Pb complexes.The emission wavelength shied from the green color to the yellow color when the functional group varied from PbN 3 Br 2 to PbN 4 O 3 .(see the Fig.10).

Table 4
The optical properties of compounds